The fender lights on a Jeep CJ are not grounded an depend on the unpowered filament in the turn signal bulb to provide a ground path. The fender light needs only a small amount of current to illuminate and restricts the current going through the unpowered filaments so they won’t illuminate. This way they will remain on steady with the running lights or flash with turn signals. If both turn signals and running lights are on then it will flash in opposite phase of the turn signal.
Quite a clever little circuit.
However when replacing the incandescent lights with LED, things begin to go astray...
